1. Field of the Invention
The present invention relates to a polishing composition suitable for planarization of the surface of semiconductor devices. More particularly, the present invention relates to a polishing composition which scarcely produces polishing scratches and which is readily cleansable after polishing, in polishing of semiconductor devices to which so-called chemical mechanical polishing (hereinafter referred to as CMP) technology is applied, in processing of device wafers for producing semiconductor devices.
2. Discussion of Background
Progress of computer products has been remarkable in recent years, and parts to be used for such products, such as ULSI devices, have been developed for high integration and high speed, year after year. Along with such progress, the design rule for semiconductor devices has been progressively refined year after year, the depth of focus in a process for producing devices tends to be shallow, and planarization required for the pattern-forming surface tends to be increasingly strict.
Under these circumstances, there has been an attempt to accomplish the planarization by polishing, and presently, such a polishing process is used as one of many device producing processes. This polishing process is commonly referred to as CMP, and the frequency of its use is expected to increase also in future.
Presently, polishing compositions used in the CMP process employ, in many cases, a slurry having fumed silica dispersed in water, and they are mainly used for planarization of insulating films.
The slurry employing fumed silica, is usually one obtained by adding an alkali such as potassium hydroxide or ammonium, to fumed silica, followed by dispersion in water. The alkali is added to disperse the fumed silica stably in water, and to achieve a high stock removal rate for e.g. insulating films. Therefore, the pH of such a slurry is usually at least 9.
However, in the conventional fumed silica-containing slurry, fine fumed silica particles sometimes agglomerated. Impurities present in the slurry are considered to be the cause for such agglomeration. The mechanism is as illustrated in FIG. 1.
FIG. 1 illustrates a case where aluminum ions are present as the impurities. The slurry is alkaline, and fine silica particles are dispersed stably in the slurry when the impurities are not present. However, when the aluminum ions are present, hydroxyl groups in water will be coordinated with the aluminum ions. Dehydration-condensation takes place between the hydroxyl groups coordinated with the aluminum ions, and hydroxyl groups on the surface of fine silica particles. As a result, silicon atoms on the surfaces of the respective fine fumed silica particles will be bonded via the impurities, and thus, agglomeration is considered to take place.
Such an agglomeration phenomenon occurs at the time of polishing when heat and pressure are generated so that dehydration-condensation is likely to take place. The agglomerates produced at that time tend to produce scratches on the surface of a wafer during the polishing and consequently deteriorate the yield of the device. Further, atoms on the surface of the wafer and atoms on the surface of the fine silica particles will be bonded via impurities during the polishing, whereby fine silica particles will be attached on the surface of the wafer. The fine particles thus attached on the wafer surface, tend to remain even after the completion of the polishing, and they may not be removed by usual cleaning. The silica particles will thus remain on the wafer surface after the cleaning and further deteriorate the yield of the device.
Further, the slurry may be subjected to filtration to remove the agglomerates. Namely, it is common to remove the agglomerates present in the slurry by filtration of the slurry by means of a filter before use of the slurry, or by providing a filtration step in its recycling line when the slurry is used by recycling. However, when the silica particles pass through the filter, dehydration-condensation may take place by the water pressure, whereby agglomeration of the particles may take place. In other words, filtration to remove the agglomerates, may, rather, end up with producing agglomerates.
From such a viewpoint, a polishing composition is desired which is substantially free from agglomeration of fine silica particles at the time of polishing or filtration and thus scarcely produces polishing scratches by agglomerates, and which is readily cleansable after polishing.
The present invention is made to effectively solve the above-mentioned problems. Namely, it is an object of the present invention to provide a polishing composition which scarcely produces polishing scratches by agglomerates and which is readily cleansable after polishing.
The polishing composition of the present invention is one comprising the following components (a) to (c):
(a) colloidal silica;
(b) at least one bicarbonate selected from the group consisting of ammonium bicarbonate, lithium bicarbonate, potassium bicarbonate, sodium bicarbonate and a mixture thereof; and
(c) water;
wherein the concentration of each of the elements included in Groups 2A, 3A, 4A, 5A, 6A, 7A, 8A, 1B and 2B, lanthanoid and actinoid, and the concentration of each element of aluminum, gallium, indium, thallium, tin, lead, bismuth, fluorine and chlorine, are at most 100 ppb in the polishing composition, respectively.
The present invention thus provides a polishing composition which scarcely produces polishing scratches by agglomerates of fine silica particles and which is readily cleansable after polishing.